RHCP in a category of their own.
Then somewhere below them, Greenday.
Then well below them, Foo Fighters.
Then well below them, but with the momentum of hype behind them, The Killers.
Pearl Jam are still a huge force, but if you are talking the combo of pull that creates and generates attention, they have dropped off a bit and I think are working mostly off an established base.

I've got a 3rd row center floor seat for RHCP next week in San Antonio and I'm passing it up. They suck live, from what I've seen, and they aren't a $60 concert ticket band. I feel like all their songs sound alike, Kiedas can't sing live, and there is very little about their music that I'd consider inspired.

I will agree that they're the biggest American band. However I'd argue that Wilco is the best American band.